Filter cartridges formed of a nonwoven mass of a melt blown polymer are well known and have achieved widespread use in fluid filtration applications. Typically, such melt blown filter cartridges are made by extruding a polymer through orifices associated with a melt blowing die to form fibers which are directed toward an axially elongate rotating perforated core element. During melt blowing, a flow of inert gas (e.g., air) acts on the molten fibers so as to attenuate the fibers to relatively fine diameter and to randomly distribute the attenuated fibers onto the core element. Over time, therefore, an annular mass of nonwoven, randomly intermingled solidified fibers builds up on the core element. Controlled axial movement of the built-up mass of melt blown fibers relative to the melt blowing die will therefore allow a cylindrical filter cartridge of indefinite length to be formed continuously.
U.S. Pat. Nos. 4,112,159 and 4,116,738 to Pall (hereinafter "the Pall '159 and '738 patents", the entire content of each being expressly incorporated hereinto by reference) disclose the temporary end-to-end joining of sequential preformed core elements by means of coaxially interdigitated spacers so that the joined preformed core elements are capable of being rotated and axially traversed as a unit relative to a melt-blowing die during the continuous production of melt-blown filter cartridges. The melt-blown fiber layer is subsequently cut at about the midpoint of the spacers leaving a lap which extends beyond the core elements at each end, thereby making it possible to pull off a filter length by withdrawing the core portion of the next following spacer. Presumably, the spacers removed from the filter lengths are then reused in the process disclosed in the Pall '159 and '738 patents.
As an alternative to using preformed core elements, the Pall '159 and '738 patents also disclose that the core element can be formed in situ by means of a continuous rotatable tubular extrusion die. The core element is thus extruded continuously in tubular form with an open central passage, in a continuous length. Prior to receiving the melt-blown fibers, the extruded core element is perforated or slit by cutting means to provide a plurality of apertures for passage of fluid therethrough into the central open passage of the core.
The techniques disclosed in the Pall '159 and '738 patents are not without disadvantages. For example, when using spacers as the means to couple preformed core elements in an end-to-end manner, care must be exercised that the melt-blown fiber media is cut at about the spacer's midpoint, thereby limiting the maximum length of the filter cartridge to the length of the preformed core element. Furthermore, the cut cannot be made entirely through the melt-blown fiber media and the spacer, since to do otherwise would result in sacrifice of the spacer thereby adding to the overall production costs of the filter cartridge. On the other hand, the continuous extrusion of the core element necessarily involves the provision of a rotatable extruder and core-perforation equipment which may not be cost effective in terms of already preformed core elements.
Thus, what has been needed in this art are apparatus and methods whereby preformed core elements may be joined integrally end-to-end without necessarily using any separate joining structure (e.g., such as spacers). It is toward fulfilling such a need that the present invention is directed.